Conventional motes are small electronic devices that sense at least an element of their surroundings and communicate information related to the element sensed. A mote may communicate just the information it has collected, or such information may be combined with that from other motes and communicated, for example, to a central device.
Conventional motes communicate with one another and/or a central device. If the device or other mote with which the mote wishes to communicate is out of range, the mote may communicate via a multi-hop communication through other motes.
Because motes may rely on a small source of stored power such as batteries, it can be desirable for a mote to conserve power. One way motes draw power is during receipt of transmissions. To keep a radio on all the time to receive any transmission can represent a substantial power drain on the mote's power supply.
One conventional way that power could be conserved is to divide the time into slots, and require the motes to leave their radios off, turning them on to listen only during some of the slots. The radios are off during other slots, conserving power. If the start of a transmission is received during one of the slots in which the mote is to be listening, the mote can keep the radio on until the entire transmission is received. A beacon is periodically transmitted to synchronize the clocks of the different motes. However, this arrangement is difficult to synchronize in a multi-hop environment.
Another way power has been conserved has been to require one mote that wishes to communicate with another, to transmit a repeating preamble that signifies that a message is forthcoming. The preamble is repeatedly transmitted throughout a period in which each device turns on their radio and listens for a preamble. If the device does not detect a preamble, it turns off its radio for one preamble period, thereby conserving power. If the device detects a preamble, the device keeps its radio on until the actual communication is received. Although this technique conserves power during periods in which no preamble is received, after the preamble is received, the device is required to keep its radio on until the actual message is received, wasting power while any subsequent preambles are repeatedly received and discarded.
Recently an IEEE communication standard known as 802.15.4 has been adopted by designers of communication products, and many conventional mote manufacturers would like to use the standard for inter-mote communication. However, the standard does not handle the conservation of power very well because it employs the preamble technique, with the problems described above. What is needed is a system and method that can use the facilities of an 802.15.4-compatible device, but can conserve power better than an arrangement in which the receipt of a preamble causes a receiving device to keep its radio on until the actual communication is received, and does not involve a time-division multiplexing arrangement.